The following one-element metals may be, among others, produced as cathode sheets by means of aqueous electrolysis.
cathode sheet orpermanent cathodematerial of themetalflakes?or starter sheet?cathode sheetchromeC/FPSmanganeseC/FPSironCPScobaltCP/SSSnickelCSS—copperCP/SSSzincCPAcadmiumCPStinCPSleadCSS—Explanations:C = cathode sheetF = flakesP = permanent cathodeSS = starter sheetA = aluminumS = stainless steel
By means of electrolysis, not only deposition from an element can be produced, but, by means of codeposition, alloys as well. It is known in the state of the art that metal in highly pure form is produced due to the deposition by electrolysis. The term “highly pure” describes the absence of any undesirable components. The metal depositions produced are practically free of undesirable metallic and nonmetallic inclusions.
Cathode sheets can be used to produce strips having a purity level that cannot be achieved by metal smelting, not even with large technical efforts: Certain impurities (e.g. selenium as far as the production of nickel is concerned) cannot be eliminated by open melting. The open melting process is associated with gas absorption during the melting process and formation of oxides. Impurities can only be eliminated and oxide formation can only be avoided by vacuum melting. Ingots produced by metal smelting, e.g. from nickel, however, are coarse-grained and interspersed with segregations; therefore, they have to be remelted.
In the production of strips by metal smelting, nonmetallic inclusions may be created in several production stages, e.g. in the melting stage, due to oxidation products that were not transferred to the slag, wall ruptures from the converter and when the melt is cast by entrained slag from the casting spout. Without using special installations (e.g. for remelting electroslag), strips with few inclusions can only be produced when the work is carried out with continuous and extreme care in all production stages. Even then, inclusions may occur that might become visible only in later processing stages, particularly during cold rolling and deep drawing, unless they are detected by special sensors. Cathode sheets, on the other hand, can be produced practically without any inclusions being formed. The deposition of nonmetallic particles on the cathode can be avoided by simple measures, such as filtration of the electrolyte, covering of anodes and cathodes with bags or application of the divided cell technology.
It is known that single cathode sheets can be rolled.
I. Production of Strips from Sheets of Identical Thickness or from Sheet Strips of Identical Thicknesses Cut from a Single Sheet.
Strips have been produced from sheets for at least 100 years. At that time, the state of the art of production technology did not allow for a direct production of strips of great length, since for that purpose hot rolling trains with several stands are required in the hot forming stage, which have been employed only since ca. 1925. Until then, overlapping sheets for the production of strips were at first riveted, later overlapping sheets were welded and finally staples of sheets were welded (U.S. Pat. No. 1,131,037) and the seam was forged (GB 727,985).
Since strips made of sheets of different thicknesses are not usable, in all of the above mentioned cases sheets of the same thickness were used. The sheets to be joined were produced as single rolled sheets by rolling thereby guaranteeing that the sheets to be joined were of the same thicknesses.
For production reasons, however, the thickness of cathode sheets differs strongly. Depending on metal and manufacturer, the thickness differences amount to between 50% and 150% of the minimal thickness (c.f. D.III below).
It was proposed (DE 2905508) to cut a sheet into strips and to join the strips of constant thicknesses thus obtained. Since the strip thus obtained is produced from one single plate, the thickness variations are quite limited. The problem of different thicknesses of the cathode sheets is thus avoided, the greater lengths, however, are traded off against the width of the strips.
The starting point of CH 64287 for which DE 2905506 is claimed as priority is the ostensibly “unfavorable form of the raw material” of cathode sheets. The plate form is to be brought into the form of a rod in a “cold” procedure, since the plate form is not supposed to be suited for direct transformation into longitudinally stretched shapes. The latter is, however, precisely the object of the method according to the invention. According to the claims of CH 642874, whole plates are also to be welded together, cut and subsequently rolled. The problems resulting from welding, coiling and rolling, in particular the handling of the different thicknesses of the cathode sheets, are not even mentioned, let alone solved.
Indeed, according to the state of the art, thickness variations between different sheets joined to form one strip cannot be controlled in rolling procedures. Therefore, strips made of cathode sheets are only available in dimensions that can be produced from one single plate.
II. Joining of Electrolytically Produced Strips and Sheets of Identical Thickness
1. Electrolytic Joining Method
The joining of electrolytically produced strips is already known. U.S. Pat. No. 2,569,368 describes an electrolytic joining method for electrolytically produced, narrow and thin strips. The method is particularly recommended for the production of endless strips. First, the ends to be connected are bent. After fixation of the sheets, the ends serve as side walls of an electrolytic cell. Not only is the cavity formed by the bending electrolytically filled with metal, but also a certain upward projection is created. Subsequent to the removal of the strip from the cell, the upward projection and the material deposited in the former depression with the sheet ends delimiting the depression are ground such that they reach the level of the strips. The method is advantageous since sheet and joining seam approximately have the same composition, due to the fact that the joining seam is produced in the same way as the sheets to be joined. In addition, the joining procedure is carried out in cold manner so that problems associated with welding processes can be avoided.
The method should is only to be used for joining thin strips; that is it is only justifiable from an economic point of view as far as the joining of thin bands is concerned. The material to be deposited is always several times thicker than the original strips, since a cavity to be filled with electrolytically deposited material has to be generated. In order to allow for bending of sheets having a thickness from 4 to 6 mm, i.e. the minimal thickness of cathode sheets, a certain bending radius is required and a part of a length of several cm has to be fixed in the bending machine in order that the sheet is not pulled out of the blank-holder during bending. Therefore, the ends of the sheets have to be shortened subsequently to folding, so as to avoid too large a cavity. The electrolytic composition of layers is a very slow process; the deposition of cathode sheets takes at least one week (nickel: 5-10 days when deposited on starter sheets, the generation of which required 1.5 to 3 days; copper: about 10 to 14 days if starter sheets are used; 5 to 14 days if permanent cathodes are used. Even more time is required for filling a cavity with a cross section exceeding the thickness of the sheets to be joined.
Strips made of thick sheets cannot be produced for reasons of time and the resulting financial efforts, particularly if the method is applied sequentially.
2. Joining by Welding
DE 2905508 and CH 642974 propose the joining of cathode sheets made of nickel by welding, in particular by butt welding or using highly pure nickel welding electrodes.
For “rods” produced from cathode sheets by cleaving, the method of choice may be flash butt welding, which is, however, not recommended for whole cathode sheets. In fact also hot strips of production width are joined by butt welding, but nickel, in particular electrolytically produced nickel is significantly harder and thus, correspondingly higher compression forces have to be applied. Therefore, commercially available flash butt welding devices are not capable of joining nickel sheets in the form of entire cathodes.
It has been proposed to carry out the welding with electrodes made of pure nickel so that the purity level of the weld seams is not reduced. To this end the electrodes are not coated. In the case of manual light arc welding, however, the coating has an irreplaceable function: by formation of a slag film, the melt is isolated from the air; by disintegration it forms an inert-gas barrier consisting of CO, CO2 and H2. Welding experiments with electrodes of pure nickel showed that a strongly porous weld seam interspersed with holes is generated. The seam is sufficient to join the sheets but the strip cannot be rolled with high front-to-back tension, since it would break at the welded joint. High front-to-back tension, however, is necessary in rolling procedures, since given the thickness variations of the cathode sheets, without tension the thick sheets that have to be more strongly reduced would become broader than thinner sheets. This is undesirable since an irregular lateral expansion only leads to waste and results in an unbalanced coil during rolling.
DE 2905508 requires a thermal structure treatment of the welded joints in order that a continuous soft material can be made. Cutting a sheet leads to a strong deformation at the cutting edge with effects on the sheet, while the adjoining sheet parts are only slightly deformed. The sheet parts are situated in the heat-affected zone during welding. For this reason and because of the high temperatures reached during welding, a coarse grain is formed there. The demand for a thermal structure treatment shows that a welding procedure of low energy density has been used. The lower the energy density of the welding procedure, the broader the heat affected zone and the more significant the structure modifications. Anyway, for different reasons, such welding procedures are not suited for the object to be solved as will be explained in detail further below.
With the state of the art represented by DE 2905508 and CH 642874, cathode sheets cannot be joined by welding without causing the formation of holes and pores.